1. Field of the Invention
This invention relates to an image reading apparatus such as a scanner, and to an image forming apparatus such as a photocopier, a printer, and a facsimile machine and, more particularly, to an image reading apparatus readable of images on a sheet conveyed.
2. Description of Related Art
In image reading apparatuses provided at, e.g., digital photocopiers, printers, facsimile machines, an ADF (automatic document feeder), serving as an sheet automatic conveying device, has been provided as to be openable to automatically feed original documents to an image reading apparatus.
As a reading method done at the image reading apparatus having the ADF, so-called “flowingly reading” has been known. Flowingly reading is a method for reading images by scanning light emitted from a lamp unit to the original documents conveyed at a prescribed speed from the ADF after moving the reading apparatus provided below the ADF at the setting position with a motor and stopping the device, and then by detecting the reflecting light with the reading apparatus.
A structure around the reading position of the image reading apparatus using such a flowingly reading method is shown in FIG. 10 and FIG. 11. In the structure shown in FIG. 10, a pair of conveyance rollers 402, 403 is provided on an upstream side and a downstream side of the reading position α, and a guide member 405 is arranged over the reading position α for forming a prescribed gap with a platen glass 404. The reading apparatus 401 is moved to the reading position α, and images are read in suppressing rate deviations during the original document conveying operation upon conveying the original document P at a constant rate over the reading position α as restricting the original document P with the gap. The reading position a is provided at a position on an upstream side of the position that the original document P contacts to the platen glass 404, and the original document P is read as slightly floated from the platen glass 404.
As shown in FIG. 11, there is also a structure in which a platen roller 406 rotatable by a drive apparatus, not shown, is provided in lieu of the guide member 405 to convey the original document as pushed to the platen glass 404 with a smaller gap than that of the guide member 405. The platen roller 406 is used as a structure preventing the original document P from floating with respect to a reading apparatus having a shallow reading depth such as a CIS (contact image sensor). The reading position β is, at that time, provided at a position that the original document P near the platen roller 406 and the platen glass 406 are in contact with each other.
As a prior art regarding the reading position in image reading apparatuses for “flowingly reading”, Japanese Unexamined Patent Publication No. Heisei 9-93,392 has been known. In Publication No. Heisei 9-93,392, a structure of a digital type image reading apparatus capable of feeding original documents with a stable optical system, is described in which the original document reading position using the charge-coupled device (CCD) is placed at a position other than the narrowest portion formed with the backup roller and the original document table. With this structure, original document is read at a position avoiding regions at which dusts on the original document are easily attached or stagnated, so that black stripes due to dusts are prevented from occurring, and so that good image reading is realized.
As described above, image reading can be done even at the reading position α at which the original document does not contact with the platen glass 404 as shown in FIG. 10 as well as even at the position β as shown in FIG. 11.
Dusts or floating dusts such as toner powders, paper powders, and rubber debris generated during original document conveyance operation, however, may be scattered and deposited on the platen glass 404 at the reading position α in FIG. 10 since the reading operation is done at a position at which the original document P does not contact with the platen glass 404. During the flowingly reading operation, these deposited floating dusts may be read as an image in a stripe shape because the reading apparatus 401 does not move.
In a meanwhile, the reading position β in FIG. 11 allows reading with high quality and is advantageous, because it is easy to focus on the targeted object since the distance from the reading apparatus 401 is constant and because the apparatus is rarely affected from impacts during the original document conveyance operation. Dusts or attached dusts such as inks, correction liquids, and pastes attached to the original document P may be easily attached to the platen glass 404 upon contacting to the platen glass 404, and therefore, the dusts may be read as an image in a stripe shape when the attached dusts are reattached to the reading position of the platen glass 404.
Thus, with any structures of conventional image reading apparatuses, it is difficult to prevent images in stripe shapes from occurring due to attachment of either floating dusts or attached dusts.